Methods have been developed for efficient characterization and cloning of rare messenger RNAs. The techniques include: the synthesis of double stranded cDNA; the covalent attachment of homopolymer "tails"; the measurement, in picamoles, of the amount of cDNA produced; and the measurement of the polyadenylated mRNA, in picamoles, in impure samples. Methods have also been devised to quantify (P32)-labeled macromolecules. These methods consume submicroliter amounts of material and yield highly precise measurements. They make use of Cerenkov radiation. A cDNA library highly enriched for a specific class of T-cell products has been synthesized and screened. Approximately a dozen clones which appear to code for a lymphokine that is not fully characterized have been identified.